Press-fit glueless bearing pivot assembly

ABSTRACT

A pivot assembly for an actuator arm of a disk drive is disclosed. The housing of the actuator arm is supported on a shaft by two or more bearings. An assembly load is applied to the housing relative to the shaft such that the assembly load is carried by only one of the bearings. The axial location of the housing relative to the shaft with the assembly load applied is observed as a reference location. The assembly load is removed. One of the races of a bearing is moved until the housing returns to the reference location. In this state the bearings have a preload which is equal to the assembly load. The preload can be achieved with none of the races being loose-fit, and there is no need for adhesive.

BACKGROUND OF THE INVENTION

This invention relates to computer disc drive actuator assemblies, andmore particularly to the bearings used to support an actuator armhousing in such an assembly.

The computer industry traditionally records information on disc drives.The disc drive normally includes one or more discs which are rotated,with information magnetically recorded on the surface of the disc alongtracks. Information is read by a magnetic transducer or head which issupported on a flex arm or gimble assembly of an actuator arm. Pivotingof the actuator arm is used to move the transducer radially betweentracks along the disc surface. It is essential that the positioning ofthe transducer at a particular track be accomplished both accurately andreproducibly, to ensure that the proper information is read by thetransducer.

There has been an ever increasing desire to minimize the space requiredby computer components and to maximize the speed with which computerscan perform their various functions. To meet these desires, disc drivedesigners have sought to decrease the distance between tracks on a disc,and to decrease the time required to accurately and reproduciblyposition the transducer. Accordingly, the accuracy of the actuator armis required to be greater and greater.

Actuator arms are generally supported by a system of ball bearingsaround a shaft, such that the actuator arm rotates about the shaft.Typically two axially-spaced conventional roller bearings are used tosupport the housing of the actuator arm about the shaft. The first stepin the typical assembly of the actuator arm housing onto the shaft is toattach the inner race of a roller bearing against a shoulder on theshaft. Second, the housing is attached around the roller bearing suchthat the outer race butts up against a shoulder on the housing. Theaxial relationship between the shaft and the actuator arm housing isthus set through attachment of this first roller bearing. Thirdly, asecond bearing unit is placed around the shaft and between the shaft andthe housing.

Commercially available roller bearings often have a small amount oflooseness or "free play" both in the axial and radial direction. Thatis, the inner race can be displaced with respect to the outer racemerely by changing the direction of axial and radial force transmittedby the bearing. However, bearing free play can be extremely detrimentalto the performance of the disc drive. In particular, radial free play ofthe pivot assembly can allow the transducer to be moved to the wronglocation on the disc, such that information is distorted, readimproperly or, worse yet, the wrong information is read.

In additional to the radial accuracy of the pivot assembly, the radialstiffness of the pivot assembly must be taken into account. The pivotassembly must have proper stiffness such that the starting, moving andstopping of the transducer can be done quickly and without significantvibration of the transducer. Similar to the free play problem, vibrationcan cause distortion or improper reading of information even if thetransducer is in the proper location. The pivot assembly/actuator armmust be stiff enough so that any residual vibration is small inmagnitude, and must further not create any critical frequencies orresonances within the band width of the servo system.

Various methods have been used to try to improve the radial performanceof the bearing mechanism in actuator arm pivot assemblies. Perhaps thesimplest method is to require a higher degree of accuracy in the designand construction of the bearing mechanism, such that the resultingbearing mechanism has a smaller amount of free play. However, increasingthe bearing mechanism precision to a sufficient level can be quitecostly, and other methods have been attempted to improve the performanceof the bearing.

Another method to improve the radial performance of a bearing mechanismis to "take up" (i.e., reduce or eliminate) the free play by applying a"preload" force in the axial or radial direction. The preload force isintended to prevent a change in the direction of the force transmittedby the bearing, such that the inner race maintains a constantdisplacement from the outer race. Accordingly, the preload force shouldnormally be greater than the forces transmitted by the bearing duringuse.

In the case of spherical ball bearings, much of the free play is causedby the smaller diameter of the balls moving within the larger diametercurvature of the raceways. Applying an axial preload will cause theballs to centralize themselves in a particular portion of the racewaysof the bearings, and eliminate both axial and radial free play. Thepreload will also cause some compression of the balls and racewaysaffecting the stiffness of the pivot assembly. If two bearing units areused, generally one bearing unit will be preloaded in one direction andthe second bearing unit preloaded in the other direction, such that thepreload forces transmitted by the bearings offset each other with noresultant force on the pivot assembly. With a proper axial preloadpresent, the pivot assembly can be used without the balls of eitherbearing leaving the particular portion of the raceways, and axial andradial free play is eliminated. However, to eliminate free play whilestill obtaining the desired stiffness and endurance characteristics ofthe bearings, it is essential that the preload force be accurately setand maintained.

In configuring a pivot assembly for axial preloading of one bearingagainst another, placement of three of the races is not critical. Thefirst three races may be attached by any method against the outside ofthe shaft or the inside of the housing. It is the force placed on thefourth and final bearing race which determines the preloaded of thebearings against each other. Accordingly, preloading is generallyaccomplished by placing a defined axial force on the fourth race afterthe other three races have been axially positioned.

One known method for placing an accurate preload force on the bearingsis to allow the fourth race to be loose-fit, either against the shaft orthe housing. The placement of the first three races is set such thatthese races can withstand a significant amount of axial force withoutmoving. A preload spring is then placed against the fourth race toprovide the desired axial preload. The spring places a known andcontrolled axially directed force on the fourth race. Because the fourthrace is loose-fit, the shaft or housing places no force on the fourthrace, and the entire preload is carried through the bearing mechanism.When the preload is transmitted through the bearing and no other forcesare present, an equal and opposite axial preload must carried betweenthe housing and the shaft through the second bearing element. Thus bothbearing elements are preloaded with the identical force applied by thespring. The spring must be adequately placed so that it will continue toprovide the preload force against the fourth race throughout the life ofthe pivot assembly.

However, the loose-fit of the fourth race causes its own problems in theradial accuracy of the bearing mechanism. The loose-fit fourth race isessentially in a bi-stable state (i.e., it is either leaning in onedirection or the other), and any force which causes the fourth race towobble between states will cause radial inaccuracy in the actuator armand transducer.

The problem of the loose-fit fourth race has been addressed by applyinglock-tight adhesive between the fourth race and the shaft or housing towhich the fourth race is attached. The assembly of a pivot assemblywhich uses adhesive is largely the same as the loose-fit assemblydescribed above. However, after the preload spring force is applied tothe fourth race, the engagement between the fourth race and the shaft iscemented by adhesive. Because the set adhesive will carry the properpreload force, the spring for a glued fourth race may be removed afterthe adhesive sets up. Alternatively, a dead weight may be used to placethe proper preload on the fourth race as the adhesive sets, and the deadweight may likewise be subsequently removed.

Using a glued fourth race eliminates the bi-stable tendency of theloose-fit fourth race described above, but also creates its ownproblems. Pivot assemblies are generally assembled in clean rooms, andthe introduction of the adhesive to the clean room tends to be messy anddifficult. It is difficult to control the application of the adhesive,both to ensure that adhesive fully extends on the necessary surfaces, aswell as to ensure that no additional adhesive is applied which couldseep out to contaminate the clean room conditions. Adhesive applicationproblems become particularly egregious if the adhesive should enter thebearing structure and prohibit the bearing from working properly.Additionally, the long term stability of the adhesive is not alwaysacceptable. If the adhesive deteriorates, the fourth race might againbecome loose, destroying the accuracy of the pivot assembly.

Accordingly, it is desired to find a method of placing the properpreload onto the bearings of the pivot assembly which will not lead tothe problems discussed. It has generally been believed that the fourthrace cannot be press-fit or otherwise rigidly attached, as there was noway to ensure placement of the proper axial preload force onto such arigid attachment.

SUMMARY OF THE INVENTION

The invention is a method to properly and accurately preload bearingsused in a pivot assembly for a disc drive actuator arm. The methodavoids a loose-fitting fourth race and its problems, and avoidsadhesives and their problems. The method involves press-fitting thefourth race to a location within free play of its bearing. An externalassembly load equivalent to the desired preload is then applied viaspring, dead weight or other method to the shaft relative to thehousing, such that the external assembly load is carried by solely theother bearing (with the fourth race bearing remaining in free play). Atthis point the axial location of the housing relative to the shaft isrecorded as a reference location. The external assembly load is removed,and the fourth race is axially moved until the housing returns to thereference location. The fourth race remains in this final locationcarrying the preload force due to the friction of the press-fit. Throughthe method of this invention, the preload carried by both bearings isidentical to the external assembly load previously applied before finalpositioning of the fourth race.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away perspective view of a disc drive with the pivotassembly of the present invention.

FIG. 2 is a top plan view of a disc drive with the pivot assembly of thepresent invention.

FIG. 3 is a cross-sectional view of the pivot assembly, taken along lineA--A of FIG. 2.

FIG. 4 is an enlarged cross-sectional view of a bearing of the pivotassembly in the axial play condition.

FIG. 5 is an enlarged cross-sectional view taken from FIG. 3 of the leftside of the upper bearing of the pivot assembly, with the axial playtaken up.

FIG. 6 is an enlarged cross-sectional view taken from FIG. 3 of the leftside of the lower bearing of the pivot assembly, with the axial playtaken up.

FIG. 7 is a cross-sectional view of the pivot assembly, taken along lineA--A of FIG. 2, shown in a schematic fixture during application of theassembly load.

FIG. 8 is a cross-sectional view of the pivot assembly, taken along lineA--A of FIG. 2, shown in a schematic fixture after final axialpositioning of the fourth race.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show disc drive 10 of a computer device. A hard disc stack12 has one or more disc(s) 14 which rotate about axis 16. As shown inFIG. 1, the disc drive 10 may be packaged in a casing 18. The casing 18serves to protect the disc drive 10 as well as to support and anchor thedisc drive 10 when placed in a computer. The disc(s) 14 carry a seriesof generally concentric tracks upon which information is magneticallywritten and read.

An E-block 20 carrying a number of actuator arms 21 is shown with apivot assembly 22 which allows the E-block 20 to pivot about axis 24.One end of each actuator arm 21 carries a transducer 26 or magnetic headmounted on flex arm 28. As can be seen, pivoting of the actuator arm 21about axis 24 will cause the transducer 26 to be transported from trackto track along the surface of disc 14. The E-block 20 is pivoted by wayof a controlled motor (not shown) such as a stepper motor or voice coilmotor. In the particular embodiment shown in FIG. 1, the E-block 20 hasan extended rear portion 27 which carries a magnetic coil section 29which operates with a controlled magnetic field for proper movement ofthe E-block 20. Workers skilled in the art will recognize that anymechanism for pivoting the actuator arm 21 is acceptable so long as itcan be controlled to stop the actuator arm 21 such that the transducer26 is located on a particular track.

The speed with which the transducer 26 can be placed on a particulartrack significantly constrains the time necessary to perform a read orwrite function of the disc drive 10. Additionally, the accuracy withwhich the transducer 26 can be placed at a particular locationconstrains the density of tracks which can be permitted in a given sizedisc 14. The speed and accuracy of transducer placement is significantlyaffected by the pivot assembly 22 of the actuator arm 21.

As shown in FIG. 3, the pivot assembly 22 includes a housing section 30of the E-block 20 mounted on a shaft 32. The shaft may be made of 300 or400 series stainless steel. The shaft 32 remains stationary and issupported relative to the casing 18 which similarly supports the discstack 12. The housing 30 is supported on shaft 32 by rolling elementbearings 34, 36. In the preferred embodiment, the rolling elementbearings 34, 36 have a nominal inner radius of 1/4 inch, and a nominalouter radius of 1/2 inch. The housing 30 may be constructed of aluminumor magnesium, for instance, and may be counterbored at the top andbottom to nominally 1/2 inch. The counterboring of the housing 30provides an upper shoulder 38 and a lower shoulder 40 to support thebearings 34, 36.

While the roller elements may be cylindrical rollers, tapered rollers,needles, etc., it is preferred that the roller elements be sphericalballs. Spherical ball bearing units suitable for this use are commonlycommercially available. The bearings 34, 36 include a number ofspherical balls 42 situated between inner races 44a, 44b and outer races46a, 46b. The outer races 46a, 46b are attached to and moved with thehousing 30 of E-block 20, while the inner races 44a, 44b are attached tothe shaft 32. The inner and outer races 44a, 44b, 46a, 46b of thebearings 34 and 36 may be made of 440 stainless steel, as may thespherical balls 42.

The inner races 44a, 44b and the outer races 46a, 46b define toroidalraceways 48a, 48b for the balls 42 to travel. The curvature of theraceways 48a, 48b and contact surface of the balls 42 permits thetransmission of both axial and radial forces through the bearing 34, 36.The balls 42 are free to rotate and move along the raceways 48a, 48b.Thus the outer races 46a, 46b (and housing 30) can rotate with respectto the inner races 44a, 44b (and shaft 32) in a relatively friction-freemanner. The friction between the inner races 44a, 44b and the outerraces 46a, 46b may further be lessened by use of a lubricant inconjunction with the balls 42. If a lubricant is used, the bearings 34,36 may further include a shield, seal or other mechanism (not shown) toconstrain the lubricant within the raceways 48a, 48b.

As shown in FIG. 4 of the bearing is in its "free play" condition, thecurvature of raceways 48 is slightly greater than the curvature of thespherical balls 42. Due to this difference in curvature, the balls 42can be slightly displaced both in the axial and radial directions. Thisslight displacement allows inner race 44 to have an amount of "freeplay" with respect to outer race 46. That is, inner race 44 can bedisplaced with respect to outer race 46 merely by changing the directionof axial and radial force transmitted by the bearing. While the bearingis in its "free play" condition shown in FIG. 4, the spherical balls donot simultaneously contact both an upper side and a lower side of theraceway 48, and no force is transmitted by the bearing. The bearingsused in the preferred embodiment nominally allow 0.004 to 0.006 inchesof axial play. The nominal radial free play corresponding to this in thebearings 34, 36 of the preferred embodiment is 0.0008-0.0011 inches.Bearings of this type are available from NMB Corporation.

The disc drive 10 will often be supported such that the disc stack 12 ishorizontal and the shaft 32 of the pivot assembly 22 is vertical, anddiscussion of the assembly of the preferred embodiment disclosed hereinwill be described with reference to this directional configuration.However, it will be appreciated that the entire disc drive 10 may beplaced sideways, upside down, or in other directional configurations,and must still be functional in these other directional configurations.Accordingly, the invention described herein is expressly not limited inany way to the particular directional orientation of the disc driveused.

FIG. 5 shows the left side of the upper bearing 36 of the pivot assembly22 with the axial play taken up, as will happen under preload. The innerrace 44b is slightly axially displaced with respect to outer race 46b.The balls 42 ride on the lower, outer side and the upper, inner side ofraceway 48b. In this configuration, the upper bearing 36 transmits anaxial force upward on the shaft 32 and downward on the housing 30. FIG.6 shows the left side of the lower bearing 34 of the pivot assembly 22with the axial play taken up, as will happen under preload. The innerrace 44a is slightly axially displaced with respect to outer race 46a,but this axial displacement is opposite of the axial displacement ofupper bearing 36. The balls 42 ride on the upper, outer side and thelower, inner side of the raceway 48a. In this configuration, the bearingtransmits an axial force downward on the shaft 32 and upward on thehousing 30.

The pivot assembly 22 may be assembled as follows. First the shaft 32 issecured in a fixture 50 (shown schematically in FIGS. 7 and 8). Then thelower bearing 34 is pressed down around the shaft 32 until the innerrace 44a butts up against a lower shoulder 52 of the shaft 32. The axialposition of the inner race 44a of the lower bearing 34 is therefore nowfixed in relation to the shaft 32. The attachment may be achieved bypress-fitting, such that the inner diameter of the lower inner race 44ais slightly (about 0.002-0.004 inches) smaller than the outer diameterof the shaft 32. With a press-fit, the interference between these twosurfaces restricts upward movement of the lower inner race 44a withrespect to the shaft 32. Alternatively, the inner diameter of the lowerinner race 44a may be slightly larger or nominally the same size as theouter diameter of the shaft, and may be secured by other means,including adhesives, screw fasteners, flange attachment and other meanscommon in the art. As can be seen, the lower shoulder 52 of the shaft 32prevents the lower bearing 34 from moving further downward. The lowerinner race 44a may not even be axially secured at all, as the preloadforce, pressing the lower inner race 44a downward against the lowershoulder 52 of the shaft 32, may sufficiently attach the lower innerrace 44a to the shaft 32.

After the lower inner race 44a is attached to the shaft 32, the housing30 of the E-block 20 is pressed onto the outer race 46a of the lowerbearing 34. Again, the lower housing shoulder 40 will axially positionthe housing 30 relative to the lower bearing 34 (and hence relative tothe shaft 32). It will be recognized that the orientation of theshoulders 40, 52 on the upper side of outer race 46a and the lower sideof inner race 44a will allow the transmission of significant axial forcebetween the shaft 32 and the housing 30 without risking any axialdislocation. When the preload is applied, it should be directed suchthat the direction of force carried by the bearing follows theorientation of the shoulders 40, 52. The outer race 46a of the lowerbearing 34 may be secured to the housing 30 by any conventional means,including those discussed above. However, it is preferred that atolerance ring 54 be placed into the interface such that there will beno axial or radial movement of the lower outer race 46a with respect tothe housing 30.

After the housing 30 is in place and positioned axially with respect toshaft 32, the upper bearing 36 may be added to the pivot assembly 22. Asthe bearings 34, 36 are generally pre-assembled, the inner race 44b mustbe placed onto the shaft 32 simultaneously with placement of the outerrace 46b into the housing 30. The upper outer race 46b may be attachedto the housing 30 by any conventional means, such that it is properlyaxially positioned. Similar to the lower outer race 46a, it is preferredthat a tolerance ring 54 be placed into the interface such that therewill be no axial or radial movement of the upper outer race 46a withrespect to the housing 30. As shown, the upper bearing 36 may be axiallypositioned by butting up the outer race 46b of the upper bearing 36against the upper shoulder 38 of the housing 30. This will allow readytransmission of axial force between the housing 30 and the upper outerrace 46b without the risk of axial dislocation.

As discussed above, the axial positions where the first three races areattached to the shaft 32 and the housing 30 are not critical. Placementof the first race sets the axial relationship between the shaft and thelower bearing, placement of the second race sets the axial relationshipbetween the lower bearing and the housing 30, and placement of the thirdrace sets the axial relationship between the housing 30 and the upperbearing. It is not until the fourth race is axially positioned that apreload on the bearings is established. Workers skilled in the art willappreciate that it is unimportant whether the fourth race is an inner orouter race, or an upper or lower race, and that the invention can beequally practiced in all these configurations.

The shaft 32 has a smooth diameter section 56 to which the upper innerrace 44b is positioned. Because there is no shoulder for the upper innerrace 44b, it may be raised and lowered throughout the smooth diametersection 56 on the shaft 32. Also, because of the lack of a fourthshoulder, all axial load must be carried by the attachment between theupper inner race 44b and the shaft 32. In the preferred structure shown,the inner diameter of the upper inner race 44b is nominally 0.002-0.004inches smaller than the diameter of the smooth diameter section 56, andthe upper inner race 44b is press-fit on the shaft 32.

When the upper inner race 44b is placed directly opposite the upperouter race 46b, no axial load is being carried by the bearings 34, 36,and both bearings 34, 36 will be configured as shown in FIG. 4. The0.004-0.006 inch axial play of both bearings 34, 36 remains the axialplay of the housing 30 with respect to the shaft 32. In this state, theradial play of the shaft 32 with respect to the housing 30, willsimilarly remain the radial play of the unloaded bearing, or0.0008-0.0011 inches in the preferred bearings described above.

At this point, the upper inner race 44b must be moved along the shaft 32such that it is slightly lower than the upper outer race 46b. The upperinner race 44a is pre-positioned or offset 0.001-0.005 inches(preferably 0.002 inches) lower than the upper outer race 46b, afterwhich the upper bearing 36 will be configured as shown in FIG. 5. Inthis pre-positioned state, the axial play of the housing 30 with respectto the shaft 32 becomes only 0.002-0.004 inches. This pre-positioning isnecessary so that the assembly load (when applied) will be carriedentirely by the lower bearing 34, while the upper bearing 36 remains inan axial play condition.

As shown in FIG. 7, an assembly load 60 is then applied to the housing30 relative to the shaft 32. The assembly load 60 should be chosen to beidentical to the desired preload of the bearings, and can be appliedwith a spring, dead weight, or other mechanism. The assembly/preloadshould be chosen to eliminate the axial and radial play in the bearingsas well as increase the radial stiffness of the pivot assembly. Theproper assembly/preload will cause some compression of the balls 42 andadjacent raceways 48a, 48b, and will cause a slight increase in thesurface area of the contact between the balls 42 and the raceways 48a,48b. For the preferred bearings discussed herein, the preferred assemblyload 60 is typically from 3-5 pounds. Over and above taking the bearings34, 36 out of their free play condition, axial compression of thebearings 34, 36 with a 5 pound load is theoretically about 0.0005inches. This number is a function of radial play, raceway curvatures,bearing size, and load applied, and assumes ideal parts, nomisalignment, no ball size or raceway variations, etc. In reality thecompression of the bearings 34, 36 due to the assembly/preload has beengreater than 0.0005 inches.

The assembly load 60 causes the housing 30 to move down with respect tothe shaft 32 with the compression of the lower bearing 34. With theassembly load 60 applied, the upper bearing 36 remains within its stateof free play (depicted in FIG. 4). In this condition, the entireassembly load 60 is carried by the lower bearing 34. The lower bearing34 will now be configured as shown in FIG. 6. With the assembly load 60applied, the axial positioning of the housing 30 is closely measuredwith respect to the shaft 32 by an indicator 58 (shown schematically).Since the lower bearing 34 is now carrying the desired preload and noother forces, the housing 30 is in its desired final location withrespect to the shaft 32. The indicator 58 may be zeroed out at thislocation. The indicator 58 measures the axial position of the housing 30relative to the shaft 32 in increments of 0.001 or less, and preferablyin 0.0001 inch increments. In future applications it may be necessary tomeasure this position even more closely.

The assembly load 60 is now removed from the housing 30, and the housing30 accordingly moves up relative to the shaft 32 due to thedecompression of the lower bearing 34. As shown in FIG. 8, the upperinner race 44b is now pressed to its final axial position, as by leadscrew press 62 (shown schematically). After about 0.002 inches ofmovement, and the free axial play will be taken up. At this point, forevery 0.0001 in. movement of the upper inner race 44b, the housing 30will move downward half that amount, since both bearings are undergoingan equal and opposite compressive load. While observing the indicator58, the upper inner race 44b is pressed down the smooth diameter section56 of the shaft 32 until the indicator 58 again zeroes out. With thepreload thus applied, the upper bearing 36 will be configured as shownin FIG. 5, and the lower bearing 34 will be configured as shown in FIG.6. The preload carded by both bearings 34, 36 in this condition is equalto the assembly load previously applied.

The fixture 50 for performing these operation should be a very stiffbearing press such that all the axial dislocation caused by theassembly/preload is within the bearings 34, 36 and the pivot assembly 22itself and not the fixture. One should be able to press the upper innerrace 44b to within 0.0001-0.0002 inches of its desired position. Thefixture 50 must prevent rotation of housing 30, hold the shaft 32securely, be stiff enough so as to have very little deflection, andprevent binding or large applied moments under the loads needed to pressa bearing 36. A 3/8 inch, 56 thread per inch lead screw press has beenfound suitable to hand press the upper inner race 44b to 0.0001 inchincrements of axial position. The fixturing device 50 should retain verylittle stored strain during press operation, and provide a large amountof mechanical advantage. This will help to prevent stick-slip as thefourth race 44b is pressed into position.

Workers skilled in the art will appreciate that this invention describesa method of providing a proper, closely monitored amount of preload onthe bearing assembly. The problems associated with a loose-fittingfourth race are avoided, as are all the problems associated withadhesives.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method for preloading bearings used in a pivotassembly for use in a disc drive actuator arm, the pivot assemblypivoting about an axis and having a housing, a shaft, andaxially-arranged first and second bearings between the housing and theshaft, the method comprising;applying an assembly load to the housingrelative to the shaft in the axial direction, such that the assemblyload is carried entirely by the first bearing; observing as a referencelocation the axial location of the housing relative to the shaft withthe assembly load applied; removing the assembly load; and moving a raceof the second bearing until the housing is at the reference location. 2.The method for preloading bearings of claim 1 and further comprising thestep of:determining a desired preload necessary to substantiallyeliminate radial play of the first and second bearing.
 3. The method forpreloading bearings of claim 2, wherein the assembly load issubstantially equivalent to the desired preload.
 4. The method forpreloading bearings of claim 3 wherein the assembly load is from 3 to 5pounds.
 5. The method for preloading bearings of claim 1 wherein thereference location is measured in increments of less than 0.001 inches.6. A method for determining the final axial position of a fourth race ofa bearing of a pivot assembly for use in a disc drive actuator arm, thepivot assembly having a shaft with an axis and a housing, the shaft andhousing being connected by axially-arranged first and second bearings,the first bearing having a first and second race, the second bearinghaving a third and fourth race, the method comprising:attaching thefirst, second and third races between the shaft and the housing; placingthe fourth race between the shaft and the housing at a pre-position suchthat the second bearing remains within axial play when axial play of thefirst bearing is taken up; applying an assembly load to the housingrelative to the shaft in the axial direction, such that the assemblyload is carried entirely by the first bearing and the second bearingremains unloaded; observing as a reference location the axial locationof the housing relative to the shaft with the assembly load applied; anddetermining the axial position of the fourth race which will place thehousing in the reference location after the assembly load is removed. 7.The method for determining the final axial position of a fourth race ofclaim 6 wherein the step of placing the fourth race furthercomprises:press-fitting the fourth race to the housing.
 8. The methodfor determining the final axial position of a fourth race of claim 6wherein the step of placing the fourth race furthercomprises:press-fitting the fourth race to the shaft.
 9. The method fordetermining the final axial position of a fourth race of claim 8 whereinthe step of attaching the first, second and third races furthercomprises:abutting the first race of the first bearing against ashoulder of the shaft; abutting the second race of the first bearingagainst a shoulder of the housing; and abutting the third race of thesecond bearing against a shoulder of the housing.
 10. The method fordetermining the final axial position of a fourth race of claim 6 whereinthe step of attaching the first, second and third races furthercomprises:press-fitting the first race of the first bearing to theshaft: press-fitting the second race of the first bearing to thehousing; and press-fitting the third race of the second bearing to thehousing.
 11. The method for determining the final axial position of afourth race of claim 6, wherein the second and third races are attachedwith a first axial distance between them, wherein the first race and thefourth race have a second axial distance between them; and the step ofplacing the fourth race further comprises:placing the fourth race suchthat the second axial distance is 0.001-0.005 inches less than the firstaxial distance.
 12. A method for assembling a preloaded bearing of apivot assembly for use in a disc drive actuator arm, the methodcomprising:attaching an inner race of a first bearing to a shaft havingan axis, the first bearing having axial play in an unloaded state;attaching a housing to an outer race of the first bearing; attaching anouter race of a second bearing to the housing, the second bearing havingaxial play in an unloaded state; press-fitting an inner race of thesecond bearing to the shaft along a uniform diameter section of theshaft, at a position such that the second bearing remains in axial playwhen axial play of the first bearing is taken up; applying an assemblyload to the housing relative to the shaft in the axial direction, suchthat the assembly load is carried entirely by the first bearing and thesecond bearing remains unloaded; observing as a reference location theaxial location of the housing relative to the shaft with the assemblyload applied; removing the assembly load; and moving the inner race ofthe second bearing along the uniform diameter section of the shaft untilthe housing is at the reference location.
 13. The method for assemblinga pivot assembly of claim 12 and further comprising the step of:securingthe shaft in a fixture for attachment of the first and second bearingsto the shaft.
 14. The method for assembling a pivot assembly of claim 12wherein the step of moving the inner race of the second bearing furthercomprisesrotating a lead screw to position the inner race of the secondbearing.
 15. The method for assembling a pivot assembly of claim 12wherein a nominal diameter of the inner race of the second bearing is0.002-0.004 inches less than a nominal diameter of the uniform diametersection of the shaft.